The present invention relates to an image forming apparatus such as a copying machine or a printer, of an electrophotographic type or an electrostatic recording type. The image forming apparatus such as a copying machine or a printer has been increasingly downsized. However, the downsizing is saturating because there is a limit in the downsizing only by downsizing each of the image forming process means such as charging means, exposure means, developing means, transfer means, fixing means or cleaning means. Untransferred toner (residual developer) remaining on a photosensitive member after image transfer operation is removed and collected by cleaning means (cleaner) into a residual toner container. The residual toner is desirably minimized from the standpoint of environmental health.
In view of the foregoing, an image forming apparatus using a cleanerless process has been put into practice, in which the cleaner is omitted, and the untransferred toner is removed from the photosensitive member by developing means (simultaneous development and cleaning), and the toner is collected into the developing means and is reused.
In the simultaneous development and cleaning process, a small amount of the toner still remaining on photosensitive member after the image transfer operation is removed in the subsequent developing steps, by application of a fog removing bias voltage (potential difference Vback between a DC voltage applied to the developing means and the surface potential of the photosensitive member). With this process, the untransferred toner is collecting by the developing means and is reused in the subsequent image formation processes. Therefore, the amount of the residual toner reduces, and the maintenance operation is easier.
Because the cleaning means is not provided, the image forming apparatus can be significantly downsized.
In such an image forming apparatus of a cleanerless type, it is desired that a measurement is taken to avoid hysteresis of the previous image. To accomplish this, it is considered that untransferred toner is temporarily collected by the charger. In order to perform the function of collecting the residual toner, an injection charger of a magnetic brush type is suitable. In an image forming apparatus using the magnetic brush injection charging, the uniformity of the charging is influenced by a thickness of a charge injection layer of the photosensitive member. As shown in FIGS. 11(a) and 11(b), the charge injected into the photosensitive member reaches an interface between the charge injection layer and a charge transfer layer. In the magnetic brush injection charging, the points of contact between the photosensitive member and the charging magnetic particles are discrete, and therefore, it is not possible to flow the electric current throughout the entire surface of the photosensitive member. However, in the charge injection layer, the charge is dispersed in the directions along the surface, thus providing a substantially uniform charged distribution as shown in FIG. 11(a). However, when the thickness of the charge injection layer is small as shown in FIG. 11(b), the lateral (in the directions along) portion of charge is insufficient with the result of deteriorated uniformity charging.
Particularly when an electroconductive brush is used and is provided between the image transfer station and the charging station and is supplied with a voltage of the polarity opposite from the toner in an attempt to improve the residual toner collection performance of the charger, stripes of latent image of the opposite polarity are formed on the surface of the photosensitive drum. The magnetic particles are not contacted to all of the latent images in the form of stripes, but they are contacted to a part of the latent images. When the thickness of the charge injection layer is small, the latent image of the opposite polarity remains. Although when the thickness of the charge injection layer is sufficient, the lateral dispersion of the electric charge provides the uniform charging to the regular polarity. In the case of the cleanerless system, there is residual toner in the charger, and therefore, the toner is discharged toward the latent image of the opposite polarity, and such toner is unable to be removed by the developing action, with the result of stripes as shown the FIG. 12 which is a sample of such an image.
If the charge injection layer is made thick to a certain extent, it is possible to avoid an increase of the stripe fog attributable to reduction of thickness due to photosensitive member scraping. However, the charge injection layer comprises fine electroconductive particles disbursed therein. Therefore, if the charge injection layer is made thicker, the amount of image light passing through the layer decreases with the result of image deterioration.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which the untransferred toner can be collected by the charger. It is another object of the present invention to provide an image forming apparatus image the toner can be returned onto the image bearing member from the charger. It is a further object of the present invention to provide an image forming apparatus capable of forming a proper length of toner transition electric field in a long term operations
According to an aspect of the present invention, there is provided an image forming apparatus, comprising an image bearing member; charging means for electrically charging the image bearing member; an electrostatic image forming means for forming an electrostatic image by selectively discharging the image bearing member charged by the charging means; a developing means for developing the electrostatic image formed on the image bearing member onto a transfer material, wherein the charging means is capable of collecting residual toner from the image bearing member after an image transfer operation is performed by the transfer means; an electric field forming means for forming an electric field between the charging means and the image bearing member to transfer the residual toner from the charging means to the image bearing member; and a control means for controlling a length of time during which the electric field forming means forms the electric field, wherein the control means controls the length of time substantially in accordance with one of a surface of the image bearing member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.